Blood is commonly described as a complex red fluid consisting of plasma with a suspension of blood cells, mainly red blood cells. In an adult the blood volume is approximately 5 liters, of which 40-50% is red blood cells. The ratio between red cells and plasma is called hematocrit. Plasma consists mainly of water with proteins, sugar, vitamins, hormones, etc. A red blood cell, or erythrocyte, consists of a thin membrane as a kind of balloon in which there is water and a high concentration of the protein hemoglobin (Hb) in various forms. Hemoglobin is complex molecule containing heme groups with iron atoms that can bind to and release oxygen and carbon dioxide in the circulation. Normally there is approximately 13-15 g hemoglobin per 100 ml blood in an adult, corresponding to 4-5 million red blood cells. Even a very small sample of blood taken from one person is a representative amount of blood cells for making measurements to determine certain properties and conditions of the blood. The hemoglobin value is a representation of the oxygen transportation capacity of the blood to other tissue and a parameter for patient diagnosis. Hemoglobin value is furthermore a primary safety and quality parameter in blood banking where blood is stored for transfusion purpose and collected as raw material for blood based industrial purposes.
Hemoglobin measurement is one of the most common diagnostic tests in the world today. Recently, development has taking place in the field of hemoglobin measurement, in that whole blood can be used as measurement material, thereby relaxing the need for hemolysation. An example of such an improved hemoglobin measuring device is disclosed in the International patent application number WO 2006/104451.
In addition to determining hemoglobin in a blood sample, other blood tests are routinely employed in primary care diagnosis. An example of such additional test is erythrocyte sedimentation rate (ESR). This ESR rate is a non-specific measure of inflammation, which is commonly used as a medical screening test. Today, the test is performed by placing anti-coagulated blood in an upright Westergren tube and the rate at which the red blood cells fall is measured in mm/h. When an inflammatory process is present, the high proportion of protein, e.g. fibrinogen and immunoglobulins, in the blood causes the red blood cells to stick to each other. The red blood cells form stacks called “rouleaux” which settle faster than individual red blood cells, thereby resulting in a higher ESR rate in the case of inflammation.
U.S. Pat. No. 6,336,358 discloses a device for measuring the sedimentation rate or erythrocytes in a sample. A test container is filled with the blood sample and is inclined and held at the specific inclination angle during the test. Light is projected into the container and the light passing through the container is electrically detected by a line sensor. The detector reading is used for determining the liquid level of the supernatant in the sample and the position of the boundary between the supernatant and the sediments. The ESR can then be calculated based on the supernatant level and the position of the boundary.